The Fiber Optic Gyroscope senses rotation based on the Sagnac effect named after the physicist who first demonstrated optical rotation sensing. The magnitude of the Sagnac effect is expressed as follows: ##EQU1## where .phi..sub.s =Sagnac phase shift
A=mean area of the closed optical path PA1 N=number of turns of Fiber PA1 .OMEGA.=rotation rate of the gyro PA1 .lambda.=wavelength of the light source in vacuo PA1 C=speed of light in vacuo
In order for a rotation sensor to be used as a gyro, it must yield a repeatable output for a given input rotation rate. By inspecting the above equation it can be seen that .phi..sub.s can vary according to .OMEGA.. This is a desirable relationship for a gyro. .phi..sub.s can also vary according to .lambda. which yields a change in .phi..sub.s that does not correspond to a change in rotation. This is not desirable for a gyro. The scale factor of the fiber optic gyroscope is commonly expressed as: ##EQU2## where SF=scale factor
Therefore, a change in .lambda. will produce a change in gyroscope scale factor, a specification which must be held to certain tolerances for a given application.
The light sources commonly used for fiber optic gyroscope applications are the Super Luminescent Diode (SLD) and the Edge Light Emitting Diode (ELED). These are chosen for their high output power (for good signal to noise characteristics), compact size, low power requirements, and broad spectral bandwidth with a nearly Gaussian wavelength distribution. The broad spectral width overcomes Rayleigh back scatter noise which would obscure the rotation signal and render the gyro useless for all but the least accurate requirements. The Sagnac phase shift is then based on the aggregate effect from the entire spectrum according to the centroid wavelength.
The centroid wavelength of a diode light source is temperature dependent. A typical wavelength change is 3.ANG./.degree.C. which for usual fiber optic gyroscope geometries, would correspond to a scale factor change on the order of 100's of parts per million (ppm)/.degree.C. For most applications, this degree of scale factor deviation due to wavelength change would be intolerable.
A popular approach for diminishing scale factor deviation due to wavelength variation, is tightly controlling the operating temperature of the diode. Diode light sources typically come with a Thermo-Electric Cooler (TEC) built into the package. The operating temperature of the diode can then be controlled via the TEC, at some temperature to within a few hundredths of a degree C.
Another approach employs a miniaturized, bulk optic interferometer inside the light source package which senses wavelength shift from some predetermined operating point. The optical signal is transformed into an electronic error voltage which is then used to servo the diode operating temperature via the TEC until null is again achieved.
Both of these approaches have a latent defect. The scale factor error due to wavelength is based on the centroid wavelength of the light as it exists in the fiber optic sensing coil. All of the optical components prior to the coil and the coil itself in general, act as wavelength filters. Due to the broad band nature of the light source spectrum, each optical component, therefore, would tend to alter the optical spectrum and thereby change the centroid wavelength of the light in the sensing coil. This alteration of the spectrum in the coil would be further compounded by each component's temperature sensitivity and would occur regardless of what was done to control the light source centroid wavelength. Although light source centroid wavelength control will help scale factor wavelength error, it does not yield the desired results due to the above optical component contribution.
Additionally, both of the above approaches have a problem with long term wavelength drift due to diode aging. For a given temperature the diode centroid wavelength will drift due to aging. The temperature control approach clearly does nothing to address this problem. The wavelength servo approach will attempt to always maintain a specific wavelength via the operating temperature of the diode. The problem with this is that the desired operating wavelength may, over time, appear at a temperature which will cause premature diode aging and failure.
The present invention addresses all of these problems and provides a novel and elegant solution.